The invention relates to a milling plant having a roller grinding mill, a classifier and an exhaust gas fan, which follows a filter, particularly a plant for the production of cement, and to a method for cement production in a compound system.
In the production of cement raw material mill drying plants are operated in combination with a clinker burning process, in order to supply the exhaust gases formed in a cement rotary kiln, following heat exchangers, to a further heat utilization.
A known milling system can be gathered from a circuit diagram according to FIG. 3. Exhaust gases 3 from a calcining process are supplied by a kiln fan 5 into an exhaust gas pipe 6, via a shutoff device 6a to a roller grinding mill 2. The term kiln fan 5 is used here to define the fan feeding a gas flow to a mill. In combined circuits of cement rotary kilns and heat exchangers with an air-swept roller grinding mill, the kiln or kiln/heat exchanger-fan supplies the kiln gas flow and heat flow to a roller grinding mill, e.g. an air swept roller grinding mill. The term mill fan is used to define the fan, which delivers the gas flow necessary for the mill and which as fan 7 is positioned in the flow direction below the filter 8 functioning as a dust separator. Said fan 7 supplies the exhaust gas 3 with fines or meal 34 produced in the roller grinding mill 2, by means of a dust-exhaust gas pipe 9, which contains a shutoff device 9a, to the filter 8. The fines 34 separated in the filter 8 are delivered by means of not shown conveying systems into silos. The dust-freed exhaust gas 3 is supplied with the aid of the fan 7 following the filter 8 to a not shown chimney flue and ejected.
In a compound operation, in which the milling plant is operated on line with a not shown cement rotary kiln, a shutoff device 15a in a bypass line 15 remains closed. Unusable thermal energy is dissipated in a cooling tower 22, which generally follows the kiln fan 5, or in the roller grinding mill 2, e.g. by water injection.
Individual components of a raw material mixture 33 are supplied from bunkers 31 by means of weighting belts 30 to a feed belt 32 and fed to the roller grinding mill 2. The feed device can be constituted by a flap lock 25, which acts as an air excluder. Coarse material 26 separated in the roller grinding mill 2 is at least partly admixed with the raw material mixture 33 by means of a conveying machinery 24 and is supplied again to the roller grinding mill 2.
In combined operation the milling plant according to FIG. 3 is operated in such a way that the pressure-zero point, i.e. the point of atmospheric pressure, is located shortly upstream of the entry of the exhaust gases into the roller grinding mill 2, so that said grinding roller mill 2, an integrated classifier 13 and the filter 8 operate under a relatively high sub-atmospheric pressure and corresponding sealing mechanisms and stable structures are required, which will be discussed hereinafter.
In direct operation the milling plant according to FIG. 3 is out of action and only the not shown cement rotary kiln is operated. In combined operation the dust produced by the mill 2 and the residual dust from the heat exchanger are separated in the filter 8, so that the term xe2x80x9cfilter plantxe2x80x9d is used. However, in direct operation only the residual dust from the exhaust gas 3 from the not shown heat exchanger is separated. The shutoff device 6a in the exhaust gas pipe 6 and the shutoff device 9a in the dust-exhaust gas pipe 9 are then closed and the exhaust gas flow 3 from the calcining process, following the cooling tower 22, is passed directly via the bypass line 15 into the filter 8, where it is dedusted and fed by means of the following fan 7 and a not shown chimney flue into the atmosphere. In direct operation the fan 7 serves as an exhaust gas fan for the heat exchanger.
For the production of cement raw material independently of the calcining process, e.g. when the cement rotary kiln is out of action or on recommissioning, as no exhaust gas is available, fresh air 4 is supplied by means of a control flap 4a and a hot gas generator 37 to the roller grinding mill 2. The shutoff devices 6a and 15a in the exhaust gas pipe 6 and bypass line 15 are then closed, whereas the shutoff device 9a in the dust exhaust gas pipe 9 between roller grinding mill 2 and filter 8 is opened.
FIG. 4 shows a circuit diagram of a further raw material mill drying system, which is also known as the xe2x80x9cthree-blower versionxe2x80x9d. For identical features the same reference numerals as in FIG. 3 are used. A first blower, which corresponds to the kiln fan 5 of FIG. 3, but which is not shown, is located in the flow direction upstream of a cooling tower 22 and feeds the exhaust gases 3 from the calcining process to a roller grinding mill 2. A second blower 28, which acts as a mill fan, is located downstream of a multiple-unit cyclone 29 and feeds a partial gas flow, via a return line 16 back into the roller grinding mill 2. The remaining exhaust gas part from the multiple-unit cyclone 29 is supplied by means of a control and shutoff device 14 to a filter 8. The filter 8 is followed by a third blower 38 as exhaust gas fan and which feeds the remaining exhaust gases 3 into a not shown chimney flue as filter exhaust gas. The fines 34, 35 separated in the multiple-unit cyclone 29 and in filter 8 are supplied by corresponding conveyer mechanisms 36 to a not shown silo. Reference is made to the remarks concerning FIG. 3 in connection with the devices for the production and supply of a raw material mixture 33 to the roller grinding mill 2.
The mill drying plant according to FIG. 4 can operate independently of the gas and thermal balance of the compound system. By means of a bypass line 15 excess exhaust gas and heat flows from the kiln and calcining process can be bypassed the roller grinding mill 2 with classifier 13 and multiple-unit cyclone 29 and can be dedusted together with the exhaust gases from the roller grinding mill 2 in the following filter plant 8.
FIG. 6 shows an exemplified pressure curve of the combined circuit variant of the plant of FIG. 4. The essential devices with the corresponding association are shown above the pressure curve and given the reference numerals of FIG. 4. FIG. 6 shows that the filter 8 operates in a relatively low sub-atmospheric pressure range and therefore only has to be protected against air infiltrations by relatively limited expenditure. However, the roller grinding mill 2, which is operated with a vacuum of about xe2x88x9250 to xe2x88x9280 mbar, must be virtually xe2x80x9chermeticallyxe2x80x9d sealed to avoid air infiltrations.
Disadvantages also result from the multiple-unit cyclone 29, which is associated with relatively high construction and space consumption costs and is subject to wear, so that there are also high maintenance costs.
A high degree of separation in the multiple-unit cyclone 29 also requires relatively high energy costs and a further disadvantage of the known plant is the splitting up of the end product into coarser fines 35 from the cyclones and finer fines 34 from the filter 8. (FIG. 4).
The flexibility of the milling plant is limited, because the separation level of the multiple-unit cyclone 29 is coupled with the load state (=gas flow) of the mill. The cyclone separation level drops in the case of a partial load, so that there is a rise in the residual dust content in the line 39 following the multiple-unit cyclone 29 to the mill fan 28, which leads to wear phenomena.
Although admittedly the mill drying plant according to FIG. 3 has a simpler pipe layout, lower energy costs and relatively low capital costs due to a compact plant construction. As can be seen from the exemplified pressure curve of the compound circuit variant of the plant according to FIG. 3 shown in FIG. 5, the filter 8 is incorporated into the vacuum range of the roller grinding mill 2, which is approximately xe2x88x9270 to xe2x88x9290 mbar and must therefore be correspondingly designed from the construction and safety standpoints. Filter housings are of considerable size in industrial milling plants. The filter housing must be designed for operational vacuums of almost xe2x88x92100 mbar and for a cold air start to xe2x88x92140 mbar for safety reasons. Considerable constructional and production measures are necessary to achieve the necessary rigidity and prevent a collapse. Even as a result of small cracks in the housing, the following mill fan 7 sucks secondary air into the plant, which acts as a loss on the mill exhaust gas and leads to functional problems.
Pressure fluctuations, which are unavoidable in the enlarged vacuum range, lead to high mechanical loading of the filter housing and to a high susceptibility to cracking and therefore air infiltrations. These directly influence the capacity of the roller grinding mill. In addition, the not xe2x80x9chermeticallyxe2x80x9d sealed points of the roller grinding mill and the classifier, together with corresponding areas in the overall system are potential sources for air infiltrations. The damage points can also not be located from the outside due to a closed insulation.
Another disadvantage is that the exhaust gases 3 must be drawn out of the heat exchanger unit of the calcining process during compound operation through the roller grinding mill 2 and a bypassing of the exhaust gases is impossible (cf. also FIG. 3).
The gas temperature in the dedusting filter 8 cannot be kept independent of the mill exhaust gas temperature. This dependence can alone and together with air infiltrations lead to a local dropping below the dew point and therefore to corrosion in the filter and pipe area. During mill operation no other gases than those from the roller grinding mill 2 can be dedusted.
The object of the invention is to provide a milling plant and a method for the milling of raw materials, particularly a cement production plant and a method, which with a particularly low capital and maintenance costs ensure a variable method performance and an extremely efficient milling process and in particular an efficient cement production.
From the apparatus standpoint, this object is achieved by a milling plant, e.g. by a plant for the production of cement with a roller grinding mill, a cement rotary kiln with a heat exchanger unit, a kiln fan and an exhaust gas fan downstream of a filter, in which a mill fan is positioned upstream of the roller grinding mill and overcomes the high resistance of the roller grinding mill blade ring and forces an exhaust gas or also air flow through the roller grinding mill blade ring necessary for a mill drying process and, in cooperation with the exhaust gas fan, permits an advantageous pressure curve upstream, within and downstream of the roller grinding mill and extending to the filter.
From the method standpoint, according to the invention the prepositive mill fan positioned upstream of the mill forces the pressure zero point, i.e. the atmospheric pressure into the roller grinding mill and operationally reliably held in the area of the blade ring and milling pan, particularly in an upper area of the blade ring and extending to an upper edge of the milling pan. In the milling chamber of the roller grinding mill above the milling pan and blade ring, including at the passages for the levers and/or spring rods, in the classifier and in the filter, a minimum sub-atmospheric pressure is set by means of the exhaust gas fan, whereas in the flow direction upstream of the blade ring, particularly in the lower part of the mill, an overpressure is produced by the mill fan.
Since, according to the invention, lower sub-atmospheric pressure levels can be set than in the known mill drying plants, it is advantageously achieved that it is possible to move away from the known, stable costly constructions of the filter or cyclones and further milling plant equipment and a considerable percentage of the air infiltration sources is eliminated.
The disposition of a mill fan upstream of a roller grinding mill is known per se from coal milling plants. However, in that case the mill fan is the sole fan in the coal milling system. In addition, the milling chamber, classifying chamber and e.g. the following burner lines and coal dust/air mixture burners are under an overpressure.
According to the invention, the upstream mill fan in a compound cement raw material milling plant with mill drying process serves to displace the pressure level in such a way that in cooperation with the exhaust gas fan in the mill interior there is a much lower sub-atmospheric pressure extending to ambient pressure. The total capacity of the fans of the mill drying plant, i.e. the mill fan and exhaust gas fan downstream of the filter system, is distributed over the fans and is lower due to the reduced air infiltrations. The lower part of the mill is under an overpressure. The filter can be operated with a much lower vacuum than in the prior art in accordance with the circuit of FIG. 3.
Thus, the construction of the filter housing for absorbing the surface pressure can be significantly simplified in avantageous manner.
The gas flow through the roller grinding mill is controlled by the upstream mill fan. It is advantageous that the control of a bypass exhaust gas flow can be brought about without difficulty by the exhaust gas fan due to the smaller pressure gradient to the filter. Due to the reduced pressure level upstream of the filter, it is also possible to connect dedusting devices to the line between the roller grinding mill and the filter.
Essential advantages of the plant and method according to the invention with a clearly defined pressure curve in the mill system are the omission of complicated seals against air infiltrations in the roller grinding mill, e.g. in the material feed area, energy savings due to the air infiltrations which no longer occurs, reduced capital and operating costs, together with flexible method performance in combination and the possibility of being able to dedust other exhaust gases in the same filter plant. A considerable cost saving more particularly results from the lighter construction of the filter housing and the omissions of cyclones.
Besides the economies in capital expenditure, maintenance costs and costs for increased energy consumption are avoided.
The invention cannot only be used with advantage for cement raw material milling plants. It falls within the scope of the invention to operate a milling plant without any upstream process, e.g. a so-called central milling plant, using an upstream mill fan, in order to displace the pressure level from the sub-atmospheric pressure to the overpressure range into the milling chamber of the roller grinding mill and to produce a vacuum by an exhaust gas fan following the filter. Considered in the flow direction, the lower milling chamber up to the suction connection of the exhaust gas fan is under a lower vacuum than in conventional milling plants.
The advantages are as in the case of a cement raw material milling plant, namely considerable energy and capital cost savings, the avoiding of air infiltrations, maintenance costs, and a variable method performance.
An inventive milling plant, in which an upstream mill fan cooperates with an exhaust gas fan, can be used for milling the most varied raw materials such as clinker, metallurgical sand, coal and all other mineral and rock types.
In the so-called filter plants, the use of a fan upstream of the mill means that there is a further fan besides the exhaust gas fan. In the hitherto conventional filter plants the fan, positioned behind the filter, acts both as a mill fan and as an exhaust gas fan. The above-described advantages of the modified pressure level (MPL) process performable with an upstream mill fan make the additional costs of a fan appear extremely small.